The invention relates to teleoperation with time-varying delay of control or feedback signals.
Teleoperation involves the manipulation and control of objects and devices over long distances. Typically, a human operator interacts with a remote environment by physically manipulating an input manipulator, such as a joystick. Control signals, such as position or velocity signals, are derived from the motion of the input manipulator and sent to a distant output manipulator, such as a robotic arm. The output manipulator physically interacts with a remote environment, for example by moving an object, according to the control signals it receives from the input manipulator. In order to improve the operator's ability to control such a remote object, or to provide a more realistic "feel" of the remote environment to the operator, feedback signals, such as forces applied by the remote environment on the remote manipulator, are sent as feedback signals to the input manipulator. The input manipulator provides these feedback signals to the operator, for example, as forces opposing motion of a joystick. Providing feedback information to an operator, and in particular providing force feedback, has been shown to greatly improve the operator's ability to perform a remote manipulation task.
However, teleoperator systems, almost by their very definition, must deal with non-zero transmission times (i.e., delays) for control signals passing between an input manipulator and output manipulator, and for feedback signals passing back to the input manipulator. When the delays are below typical human reaction times, the delays are "transparent" to the operator and do not affect the operator's ability to manipulate the remote environment. However, significant signal delays can occur due to communication over very large distances (e.g., satellite radio channels), due to communication through media with slow signal propagation rates (e.g., underwater acoustic channels), or due to communication using systems that inherently introduce delays (e.g., buffered digital systems).
A virtual reality system is very similar to a teleoperator system except that the remote environment is simulated rather than physical. An operator physically interacts with an input manipulator, such as a joystick, but no physical output manipulator is used. A typical system of this type involves a computer game providing multimodal feedback to the operator including physical (e.g., force) feedback, and visual and aural feedback. A simulation computer performs the role of (i.e., simulates) the output manipulator and remote environment. Also, multiple operators can interact with a single simulated environment. If an operator is distant from the simulation computer, control and feedback signals must be communicated between the operator's computer and the simulation computer. If such signals are sent over a data network, significant transit delays can occur.
It is well known that signal delay in the feedback loop of a feedback control system can introduce instabilities unless explicitly compensated for. This is particularly true of force-reflecting systems with delays in the forward or feedback paths.
In order to preserve stability of a teleoperator system in the presence of a fixed delay in both a forward path and a feedback communication path, a method has been previously disclosed in which signals defined in terms of "wave variables," rather than velocity and force variables, are transmitted over these communication paths. Transmission of wave variable signals in a telerobotic force-reflecting feedback system is described in U.S. Pat. No. 5,266,875, Telerobotic System, issued on May 23, 1991 to J.-J. Slotine and G. Niemeyer, referred to herein as the "previous patent". Various related configurations dealing with fixed delay transmissions between the master system and the slave system are also described by G. Niemeyer in Using Wave Variables in Time Delay Force Reflecting Teleoperation, a Ph.D. thesis submitted to the Massachusetts Institute of Technology Department of Aeronautics and Astronautics in September 1996, and referred to herein as the "Niemeyer thesis". Both the previous patent and the Niemeyer thesis are incorporated herein in their entirety by reference.